A lithium battery, which also called a lithium secondary battery, uses a solid polymer electrolyte, a gel polymer electrolyte or a liquid electrolyte, and generates a current by the migration of lithium ions. Active substances forming the positive and the negative electrode contain a polymer. A lithium-ion battery as a representative battery will be described.
A lithium secondary battery includes a positive electrode (aluminum or nickel), a positive electrode active layer (a metal oxide, carbon black, a metal sulfide, an electrolyte or a positive electrode polymer, such as polyacrylonitrile, electrolyte), electrolyte layers (a carbonate electrolyte, such as propylene carbonate, ethylene carbonate, dimethy carbonate or ethylene-methyl carbonate, an inorganic solid electrolyte, such as a lithium salt, or a gel electrolyte), a negative electrode active substance (lithium, an alloy, carbon, an electrolyte, a negative electrode polymer, such as polyacrylonitrile), a negative electrode (copper, nickel, a stainless steel), and a package containing those components.
Lithium batteries are used on personal computers, portable remote terminals, such as portable telephones, PDAs and the liken, video cameras, electric vehicles, energy storage batteries, robots, artificial satellites and the like.
The package of the lithium battery is a cylindrical or parallelepipedic metal can formed by press-working a metal blank or a pouch formed by processing a laminate structure, such as a composite film formed by laminating a film and a metal foil.
Such packages for lithium batteries have the following problems. The metal can is relatively hard and hence the shape of the battery is dependent on that of the metal can. Consequently, an external structure that uses the battery needs to be designed so as to conform to the shape of the battery, the dimensions of the external structure are dependent on the battery, and free choices in designing the external structure are reduced.
Therefore, there is a tendency to use pouches as packages. The package is formed from a laminated sheet having, in view of physical properties required of the lithium battery, workability and economical requirements, at least a base layer, a barrier layer, heat-sealable layer, and bonding resin layers for bonding together the adjacent layers. When necessary, the laminated sheet further has an intermediate layer.
Such a package for a lithium battery is a pouch formed from such a laminated sheet or an embossed package having a battery holding part formed by press-working a laminated sheet. A lithium battery module is put in the package, and peripheral parts of the package are heat-sealed hermetically to complete a lithium battery.
A part of the heat-sealable layer must be capable of being bonded to another part of the same by heat-sealing and of being bonded to metal leads extending from the battery module. Therefore, an acid-modified polyolefin resin adhesive to metal members is used for forming the heat-sealable layer.
The workability of an acid-modified polyolefin resin, as compared with that of general polyolefin resins, is low and an acid-modified polyolefin resin is expensive for forming the heat-sealable layer of the package. Therefore, a method uses a general polyolefin resin layer as a heat-sealable layer for a package, and places a lead-insulating film capable of being bonded to both the heat-sealable layer and a lead between the heat-sealable layer and the lead.
Possible lead-insulating films are those of unsaturated carboxylic acid graft polyolefin resins, metal-crosslinked polyethylene resins, and copolymers of ethylene or propylene, and acrylic acid or methacrylic acid.
When the heat-sealable layer of the laminated sheet forming a package for a lithium battery is formed of a polyethylene resin, the following problems arises when a lithium battery module is put in the package, lead-insulating films are interposed between leads and the package, and a peripheral part of the package is sealed hermetically. If the lead-insulating film is, for example, a single-layer film of an acid-modified polyethylene resin, parts of the heat-sealable layer and parts of the lead-insulating films corresponding to the leads melt when heat and pressure are applied thereto for heat-sealing and, sometimes, parts of the heat-sealable layer and the lead-insulating films are extruded outside pressed regions. Consequently, an aluminum foil serving as the barrier layer of the package comes into contact with the metal leads to short-circuit the lithium battery module.
Similarly, when the heat-sealable layer of the laminated sheet forming the package is formed of a polypropylene resin, it occurs sometimes that an aluminum foil serving as the barrier layer of the package comes into contact with the metal leads to short-circuit the lithium battery module even if single-layer lead-insulating films of an acid-modified polypropylene resin are employed.